Why use Facade pattern for EJB session bean

Question

I want to ask what is the reason to use Facade Pattern when access EJB Session Bean. In my Netbeans 6.9.1, if I do New > Sessions Bean for Entity Classes<

Answer 1

I think you should have one AbstractFacade for typical CRUD operations as you can see and many SpecificFacade for manipulating a given entity. So you can not reimplement the same basic logic many times just one... and concentrate only on more complex logic (transactions) associated with entities.

Answer 2

Yes and no. The Facade pattern makes a lot of sense, but having a separate facace per domain object makes no sense.

You will want to group facades per groups of functionality of domain objects. Imagine a billing system. It has bills, items, customer, addresses. So you would there perhaps a Facade for bill handling (adding items, setting customer, printing, marking as paid) and a different facade for creation and update of users, associating with addresses and so on.

Answer 3

The point of using EJBs at all is that they provide features such as declarative transactions and security via annotations (or XML configurations). If you don't use those features, there is no point in having EJBs.

Additionally, that autogenerated facade code is jsut a starting point (and a bad one). The EJBs should form a domain API, not a wrapper for all individual CRUD operations. They should only contain the operations you actually want to be performed on your domain model, and many of them should span several CRUD operations that need to be performend within a transaction. For example:

@TransactionAttribute
public void transferUser(User u, School from, School to){
    from.getUsers().remove(u);
    to.getUsers().add(u);
    u.setSchool(to);
    getEntityManager().merge(from);
    getEntityManager().merge(to);
    getEntityManager().merge(u);
}

The app server will execute all operations within a transaction, which ensures, for example, that you cannot have a situation where an exception is thrown for some reason and you end up with User that is removed from one Schoold but not added to the other one.